FIG. 1 shows a cross-sectional view of a conventional thin-film EL device. The conventional device has a glass substrate 11, transparent electrodes 12 in stripes formed on the glass substrate 11, a lower dielectric layer consisting of a silicon oxide (SiO.sub.2) film 13 and a SiN film 14, formed on the glass substrate 11 having the transparent electrodes 12, a luminescent layer 15 formed on the lower dielectric layer by an electronic beam deposition method, an upper dielectric layer consisting of a silicon nitride (Si.sub.3 N.sub.4) film 16 and an aluminum oxide (Al.sub.2 O.sub.3) film 17, formed on the luminescent layer 15, and Al back electrodes 18 in stripes formed on the upper dielectric layer in such a manner as to intersect the transparent electrodes 12. The upper and lower dielectric layers are formed by a sputtering method. Voltage is applied between the transparent electrodes 12 and the back electrodes 18 so as to make the luminescent layer 15 emit light and to derive the light toward the glass substrate 11.
Generally, use of the sputtering method for forming films lowers mass productivity because:
(i) Film formation speed is slow. PA1 (ii) A film formation area depends on a size of a target, and therefore is limited.
Therefore, the conventional thin-film E1 device whose dielectric layers are formed by a sputtering technique cannot offer a good productivity.
Recently, in order to solve the problem, an attempt has been made to form upper and lower dielectric layers by a plasma CVD method offering a good productivity. In this attempt, it has been tried to form a Si.sub.3 N.sub.4 film, which has a great influence on EL characteristics, by using SiH.sub.4 as material gas, specifically as reaction gas.
If a Si.sub.3 N.sub.4 film is formed on a luminescent layer by a plasma CVD method, however, an emission-start voltage Vth changes by aging, resulting in a low reliability. The change in the emission-start voltage Vth depends on a film quality of the Si.sub.3 N.sub.4 (referred to as SiN:H hereinbelow) film.
Moreover, the conventional thin-film EL device shown in FIG. 1 has the following problem.
Because the upper dielectric layer 16 is transparent and the back electrodes 18 are of Al, light emitted from the luminescent layer 15 toward the back electrodes 18 is reflected by the back electrodes 18. Reflected light passes the luminescent layer 15 onto the glass substrate 11. Accordingly, the conventional thin-film EL device does not offer display of good contrast.
In order to solve this problem, application of a brown amorphous silicon (a-Si) film into the upper dielectric layer 16 has been tried. However, if such an a-Si film is formed at a back-electrode-side surface of the upper dielectric layer 16 without any special consideration, dielectric breakdown mode of the device is apt to be of a propagate type, and luminance versus applied voltage (referred to as luminance-voltage hereafter) characteristics are not acceptable or practicable.